Silicon nitride based materials, which have been hot pressed or sintered to a ceramic, have been recognized for their heat resistant qualities useful in structural members and in some cases for use as a cutting tool. However, silicon nitride, as a single substance, is not easily sintered, even under pressure, without the addition of sintering aids. Sintering aids are substances that form secondary compounds with silicon nitride or with silicon oxide present on the surface of the silicon nitride granules, which compounds form an intergranular binder assisting in the achievement of full density and greater strength properties under ambient conditions.
With known processing techniques, it has been recognized that the substances formed by such sintering aids are harmful to high temperature use of the base ceramic since the compounds are amorphous or glassy in nature.
One attempt by the prior art (see U.S. Pat. No. 4,046,580) to eliminate the glassy phase has consisted of stripping or eliminating the silicon oxide coating on the silicon nitride granules thereby forcing any chemical reaction with the pressing aid to be with the silicon nitride. The resulting secondary phase of this attempt tends to be less glassy, but unfortunately is less useful to cutting tool applications. More useful phases would be silicon oxynitrides such as Y.sub.10 Si.sub.7 O.sub.23 N.sub.4, or Y.sub.1 Si.sub.1 O.sub.2 N.sub.1. These useful secondary phases, which are formed as a result of a chemical reaction between elements of the ternary system Si.sub.3 N.sub.4.Y.sub.2 O.sub.3.SiO.sub.2, make processing more economical and promote enhanced strength and thermal shock properties for a ceramic material that is to be used for cutting of cast iron.